The present invention relates to a focus detection device employing a pupil separating phase difference method, applicable to optical devices such as SLR (single lens reflex) cameras.
Recently, an SLR camera provided with a multi-area focus detection unit has been developed. The multi-area focus detection unit is capable of detecting a focusing condition of a plurality of focus detection areas. An example of such a multi-area focus detection unit is configured as follows.
A plurality of focus detection areas are defined on a plane optically equivalent to a film plane (i.e., a focal plane of the photographing lens). Light from an object is passed through the focus detection areas, which are arranged on and off axis positions, are divided using separating masks, respectively. The divided (pupil-divided) light fluxes are incident on corresponding line sensors, respectively, so that intensity distribution on the line sensors vary depending on the focusing condition of the photographing lens.
In the above-described type of focus detection unit, the light fluxes passed through the focus detection areas are deflected in a direction perpendicular to the pupil-dividing direction using a deflector. The separator optical system including the separator masks and the separator lenses is arranged such that the consistency with respect to the pupil of the photographing lens is maintained by deflecting the light fluxes using the deflector. With this configuration, an optical path length of the light fluxes in each focus detection optical system is maximized within the limited space, and the optical systems may not physically interfere with each other.
In the conventional focus detection unit, the deflector for each optical system is valid only in a direction perpendicular to the pupil-dividing direction. Accordingly, considerable coma is generated in a direction perpendicular to the pupil-dividing direction and the intensity of light illuminating the line sensor is lowered. Further, in the multi-area focus detection unit, in which a plurality of line sensors are arranged in parallel, a light beam incident on a certain line sensor may serve as a stray beam for another line sensor, which lowers an accuracy of the focus detection.
However, if the coma in the direction perpendicular to the pupil-dividing direction is lowered, the size of the spot in the pupil-dividing direction increases, and does not match the size of the line sensor.
In view of the above, it is therefore an object of the invention to provide an improved focus detection device in which separator optical systems are shifted in a direction perpendicular to the pupil-dividing direction, and the coma of a secondary focal plane is sufficiently corrected.
For the above object, according to the invention, there is provided a focus detection device for a camera having a photographing lens, which is provided with a pupil dividing system that divides a pupil of the photographing lens into a pair of focus detection areas, an intensity distribution pattern forming system that receives light passed through the focus detection areas and forms a pair of intensity distribution patterns, the relative position thereof varying depending on a focusing condition of the photographing lens, a plurality of light receiving elements arranged in a direction where the pupil is divided, a focus detection area defining system having a focus detection opening, the focus detection area defining system being arranged on or in the vicinity of a plane equivalent to a focal plane of the photographing lens, the light passing through the pair of focus detection areas passing through the focus detection opening, a deflection prism arranged between the focus detection opening and the pupil dividing system, the deflection prism deflecting the light passed through the focus detection opening in a direction perpendicular to the pupil dividing direction and an optical axis of the photographing lens, and an optical element provided in the vicinity of the pupil dividing system, the optical element having an anamorphic refractive power, a refractive power in the pupil dividing direction being greater than a refractive power in a direction perpendicular to the pupil dividing direction.
In a particular case, the deflection prism may function as the optical element, the deflection prism being formed such that one surface has the anamorphic refractive power
Optionally, the deflection prism may include a first prism arranged in the vicinity of the focus detection opening and a second prism arranged in the vicinity of the pupil dividing system, at least one of a light receiving surface and a light emerging surface of the second prism being formed with the surface having the anamorphic refractive power.
Further optionally, the optical element may include the intensity distribution pattern forming system, a surface having the anamorphic refractive power being formed on the intensity distribution pattern forming system.
In this case, the intensity distribution pattern forming system may have a light receiving surface and a light emerging surface, the surface having the anamorphic refractive power being formed on at least one of the light receiving surface and the light emerging surface.
Still optionally, the optical element may be an element different from the deflection prism or the intensity distribution pattern forming system, the optical element having a light receiving surface and a light emerging surface, the surface having the anamorphic refractive power being formed on at least one of the light receiving surface and the light emerging surface.
In a particular case, the optical element is disposed between the deflection prism and the pupil dividing system.
Further optionally, the optical element may be disposed between the pupil dividing system and the intensity distribution pattern forming system.
Yet optionally, the optical element may be provided with a cylindrical surface whose generatrix extends in a direction perpendicular to both a pupil dividing direction and an optical axis of the photographing lens, the cylindrical surface generating the anamorphic refractive power.
Further optionally, the optical element may have the anamorphic refractive power, and the optical element, the pupil dividing system and the intensity distribution pattern forming system are closely adhered.
According to another aspect of the invention, there is provided a focus detection device for a camera having a photographing lens, which includes a plurality of pupil dividing systems, each of the plurality of pupil dividing systems dividing a pupil of the photographing lens into a pair of focus detection areas, a plurality of intensity distribution pattern forming systems each of which receives light passed through corresponding focus detection areas and forms a pair of intensity distribution patterns, the relative position thereof varying depending on a focusing condition of the photographing lens, a plurality of light receiving elements elongated in a direction where the pupil is divided, the plurality of light receiving elements receiving the plurality of intensity distribution patterns formed by the plurality of intensity distribution pattern forming systems, respectively, a focus detection area defining member having a plurality of focus detection openings, the focus detection area defining member being arranged on or in the vicinity of a plane equivalent to a focal plane of the photographing lens, the light passed through the detection areas passing the focus detection openings, a plurality of deflection prisms respectively arranged between the focus detection openings and the pupil dividing systems, each of the deflection prisms deflecting the light passed through one of the focus detection openings in a direction perpendicular to the pupil dividing direction and an optical axis of the photographing lens, and an optical element provided on or in the vicinity of each of the pupil dividing systems, the optical element having an anamorphic refractive power, a refractive power of the optical element in the pupil dividing direction being greater than a refractive power in a direction perpendicular to the pupil dividing direction.